The present invention relates to electrical defibrillation, including cardioversion, and more particularly to body tissue stimulation electrodes, implanted subcutaneously in the thoracic region, for delivering cardioversion/defibrillation pulses.
Defibrillation is a technique employed to counter arrhythmic heart conditions including tachycardias, flutter and fibrillation of the atria and/or the ventricles. Typically, electrodes are employed to stimulate the heart with electrical impulses or shocks, of a magnitude substantially greater than pulses used in cardiac pacing. One approach to defibrillation involves placing electrically conductive paddle electrodes against the chest of the patient. During cardiac surgery, such paddles can be placed directly against the heart to apply the necessary electrical energy.
More recent defibrillation systems include body implantable electrodes. For example, patch electrodes can be applied directly to epicardial tissue. Electrodes can be placed at the distal end region of an intravascular catheter inserted into a selected cardiac chamber. U.S. Pat. No. 4,60,705 (Speicher et al) discloses an intravascular catheter with multiple electrodes, employed either alone or in combination with an epicardial patch electrode. Compliant epicardial defibrillator electrodes are disclosed in U.S. Pat. No. 4,567,900 (Moore), and in U.S. Pat. No. 5,105,826 (Smits et al).
Epicardial electrodes are considered efficient in the sense that less pulse generator energy is required for delivering effective defibrillation pulses. However, epicardial electrode implantation typically requires highly invasive major surgery including entry of the chest cavity, typically involving the spreading of adjacent ribs or splitting of the sternum, presenting a risk of infection. Epicardial electrodes must either be quite small, or highly compliant and resistant to fatigue, as they must maintain a conformal fit to the epicardium.
Larger defibrillation electrodes usually are preferred, since increasing the electrode size reduces impedance at or near the electrode. Sensing artifacts also are reduced. However, the larger electrodes are more difficult to attach to the epicardium and are more subject to fatigue, as they must conform to the heart during contractions associated with normal cardiac activity.
Subcutaneous defibrillation electrodes are easier to implant, and the implant procedure involves less risk to the patient. To achieve a large effective electrode area and yet maintain patient comfort to the extent possible, defibrillation can be accomplished with an array of several electrodes or electrode segments spaced apart from one another, as disclosed in U.S. Pat. application Ser. No. 07/533,886 filed Jun. 6, 1990, assigned to the assignee of this application. The electrode segments are relatively long and narrow, and are coupled to a single electrically conductive lead for simultaneous delivery of the defibrillation pulse. The electrode segments are highly compliant, and can be formed of composite conductors in the form of titanium or tantalum ribbons or wires, or a cable consisting of a silver core in a stainless steel tube. In either case, an outer layer of platinum can be applied by sputtering, or cladding after applying a Tantalum layer for adhesion. A braided construction also can impart compliance to an elongate electrode or electrode segment, as disclosed in U.S. Pat. No. 5,005,587 (Scott).
In view of the greater efficacy of large effective shocking area electrodes, longer electrodes and electrode segments are preferred. However, as an electrode of a given diameter becomes longer, its impedance increases, as does the voltage drop from the electrode proximal end to the electrode distal end. This leads to an unwanted gradient in interelectrode current distribution. Typically the conductive path is substantially longer than the linear length dimension of the electrode, because of the common practice of configuring electrodes as helically wound coils to enhance their fatigue resistance and ability to continually conform to body tissue. Of course, the diameter of the strand forming the coil electrode can be increased to improve its conductivity. However, this also increases electrode stiffness and patient discomfort. A further constraint arises from the fact that the electrode is in surface contact with body tissue. Accordingly, electrode materials must be body compatible as well as electrically conductive. Certain materials with highly favorable conductivity and fatigue properties are unavailable for use in such electrodes.
Therefore, it is an object of the present invention to provide an elongate electrode that is highly conductive in spite of a relatively small diameter and lateral cross section.
Another object is to provide a body implantable tissue stimulating electrode of considerable length, yet substantially uniform in potential (voltage) level from its proximal end to its distal end.
A further object is to provide an electrode structure incorporating a primary conductive path in contact with body tissue, and a conductive shunt path with a fluid tight sealing means preventing the shunt path from contacting body fluids or body tissue.
Yet another object is to provide a cardioversion/defibrillation electrode array having a high degree of redundancy.